1. Field of the Invention
This invention relates to a porous ceramic article, the pores of which are in communication with one another. The article may be used as a filter, particularly for molten metals, for example, aluminum.
2. Description of the Prior Art
U.S. Pat. No. 3,939,079 discloses a filter medium which can be prepared by blending one hundred parts by weight of aggregate particles of at least one of fused alumina, sintered alumina, silicon carbide, and silicon nitride, 5-18 parts by weight of an inorganic bonding material having a chemical composition of about 10 to about 50 percent by weight of SiO.sub.2 and about 5 and about 20 percent by weight of B.sub.2 O.sub.3 and containing a major proportion of non-vitreous substance, and about 3 to about 15 parts by weight of a combustible substance such as grain powder, coke powder or an organic resin powder, kneading the mixture with necessary amounts of a caking material and water, molding the kneaded mixture, drying the molded, kneaded mixture and thereafter calcining the dried mixture at a temperature not lower than about 1100.degree. C., during which time a number of air permeable pores of constant size are formed as a result of burning a combustible substance or the evolution of decomposition gases. The size and number of air permeable pores present in such filter medium are controlled by adjusting the amounts of bonding material and combustible substance, rather than relying on particle size of the aggregate as in the production of conventional filter media using a vitreous bonding material.
U.S. Pat. No. 3,097,930 discloses a method of making a porous shape of sintered refractory material by impregnating a sponge shape with a suspension or dispersion of refractory particles in a liquid followed by removal of the liquid and material of the sponge by heating in an inert atmosphere to break down and volatilize the sponge material and thereafter further heating to sinter the refractory particles together.
U.S. Pat. No. 3,947,363 teaches that porous ceramic foam material particularly useful in filtering molten metal can be prepared by impregnating an open cell flexible organic foam material with an aqueous slurry of ceramic material. The impregnated sponge is thereafter compressed to expel a portion of the slurry while leaving the web coated with slurry. The impregnated sponge is thereafter released from compression so that the web remains coated and the material is dried. The dried material is then heated to first burn out the flexible organic foam and then to sinter the ceramic coating thereby providing a fused ceramic foam having a plurality of intricate connected voids surrounded by a web of bonded or fused ceramic in the configuration of the flexible foam. This procedure is fully described in U.S. Pat. No. 3,893,917.
U.S. Pat. No. 4,024,212 describes the preparation of ceramic foams by the impregnation of open-celled organic polymer foam material possessing a predetermined permeability and resilience with an aqueous slurry of a thixotropic ceramic composition by passing the foam at least twice through preset rollers thereafter drying the resulting material and heating to remove the organic foam component.
U.S. Pat. No. 4,056,586 describes a method of preparing a molten metal filter by impregnating an open cell hydrophilic flexible organic foam material with an aqueous ceramic slurry, thereafter compressing the foam material to expel a portion of the slurry while leaving the web-coated therewith, releasing the compression so the web remains coated with slurry, drying the coated material, and heating the dried material to first burn out the flexible organic foam and then sinter the ceramic coating to provide a new ceramic foam having a plurality of interconnected voids surrounded by a web of fused ceramic in the configuration of the flexible foam.
U.S. Pat. No. 4,307,051 describes a process for manufacturing a light-weight refractory product by taking a particulate pore-forming material or a mixture of refractory raw material with a particulate pore-forming material, providing the individual particles of the pore-forming material or agglomerates of particles or agglomerates of the mixture with a wet exterior, coating said wet exterior with dry refractory raw material and firing the aggregate so produced. Expanded polystrene in the form of hollow spheres is taught to be a desirable particulate combustible material. The process described results in the production of a loosely sintered aggregate that is easily broken down into individual hollow particles, the vast majority of which are in the form of small hollow spheres.